As used herein the expression "fluid" means any gas or liquid or any flowable solids such as powdered or particulate solids.
The invention, while being applicable to the situations mentioned above and others, will be described by way of example only with reference to the detection of the level of an interface between a gas and a liquid within a container. In particular, the invention will be described by way of example with reference to the detection of the level of liquid in a container where the liquid in the container is liquified carbon dioxide (CO.sub.2) and when the container has CO.sub.2 gas above the level of the liquid.
Carbon dioxide (CO.sub.2) in its gaseous form is used in large quantities in many industries. One of the major consumers of CO.sub.2 is the hotel trade in the provision of draught carbonated beverages and carbonated beverages through a reticulated supply system. Other consumers also often require CO.sub.2 to be supplied in cylinders or tanks.
It has been the practice to provide some means to identify or detect the level of the liquid CO.sub.2 within the cylinder and hence the quantity of liquid remaining in the cylinder. Since, in such situations the cylinder is under high pressure it is not convenient or appropriate to employ sight gauges or the like to provide an indication of liquid level. With such cylinders containing CO.sub.2 in liquid form it has been the practice to provide an electro-mechanical level detecting mechanism employing a probe positioned within the cylinder and which probe had a plurality of sensors such as reed relays along the length of the probe. A float was associated with the probe and that float included a magnetic element for actuation of the reed relays. As it is difficult to completely remove moisture from bulk liquified industrial gases such as propane, butane, natural gas, CO.sub.2, nitrogen and the like, there is a tendency for ice particles to accumulate on the surface of the liquid gas particularly when the liquid is cooled to below freezing by adiabatic expansion. In some cases, bulk liquid gases are stored at temperatures less than 0.degree. C. With mechanical and electromechanical liquid level detectors, it has been found that accumulation of ice particles on the surface of the liquid gas frequently jams mechanical devices such as floats leading to quite unreliable operation of the level detector. In addition, where the storage cylinder was made of steel, fine rust particles have been found to accumulate at the liquid level and adversely affect the level sensing probe.
Liquid level sensing arrangements of a mechanical or moving nature were undesirable for reasons expressed above, were unreliable and therefore not accurate. Difficulties have also been encountered with prior art liquid level detectors for use in determining the position of an interface between fluids of differing specific gravities. Fermentation and distillation vessels typify situations wherein the positions of one or more liquid/liquid interfaces may be required to be determined for the purpose of controlling continuous processes.
A typical cylinder arrangement for CO.sub.2 is illustrated as part of the schematic view shown in FIG. 4 of Australian Patent Application 67304/87. Such cylinders are provided with a heating element within the tank and adjacent the bottom of the tank, a refrigerant evaporator within the tank and at an upper end thereof, a liquid level detection device extending into the cylinder and having an elongate rod and a captive float and take off and inlet lines.
Other fluid level sensing and monitoring means are described in International Patent Application Number PCT/AU85/00265; U.S. Pat. Nos. 3,911,744, 2,963,908, 4,203,325; and Australian Patent Number 493224 and 407006.
U.S. Pat. Nos. 3,911,744 and 4,203,325 are concerned with vertically spaced electrodes which when immersed in a conducting liquid, close an electrical circuit with a laterally spaced reference electrode.
U.S. Pat. No. 2,963,908 is concerned with the measurement of levels in fluids including flowable particulate solids and utilizes capacitance measurement to detect changes in dielectric constant of a surrounding medium.
Australian Patent Number 493224 described a rod-like electrode structure comprising alternating conducting and insulating sections for liquid level a measurement or pump control. The electrode requires for its operation to be immersed in a conducting liquid.
Australian Patent Application Number 407006 described a plurality of spaced thermistors operating at a normal temperature of about 300.degree. C. Immersion of a heated thermistor in a liquid changes its electrical resistivity and consequently the operating characteristics of a transistor operatively connected to the thermistor to switch the transistor between a nonconducting state and a conducting state.
International Patent Application Number PCT/AU85/00265 describes an electrode assembly similar to that described in Australian Patent Number 493224 except that the spaced conducting electrodes are discontinuous about the peripheral surface of the electrode body.
International Patent Application Number PCT/AU90/00017 describes an assembly of temperature dependent level sensors and a reference sensor. The level sensors are arranged or embedded within the probe and having a portion of the sensor capable of coming into contact with the liquid gas within the cylinder. The sensors in such an arrangement were prone to damage which was likely to occur during removal and replacement of the probe during construction, servicing or filing of the cylinder. In addition a reference sensor was necessary for the proper operation of the assembly.
In PCT/AU90/00017 the assembly employed transistors as sensors to detect changes in temperature resulting from a change in detection of liquid or gas within the cylinder. With liquid CO.sub.2 typically the temperature involved is about -25.degree. C. while gaseous CO.sub.2 may be at a temperature of between -12.degree. C. to -25.degree. C. Thus the difference in temperature between gas and liquid was sometimes slight and difficult to accurately detect with sensors of the type disclosed in this earlier patent specification.
It is an object of the present invention to provide a fluid level sensor which at least alleviates or minimises some of the disadvantages referred to above.